The present invention relates to binders for powder coatings, and to processing powder coatings that contain the binders.
EP 410 242 discloses polyurethanes containing (meth)acryloyl groups, which are prepared by reacting
(A) from 40 to 80 parts of an organic polyisocyanate with PA1 (B) from 15 to 50 parts of a (meth)acryloyl-containing monohydric alcohol and PA1 (C) from 2 to 20 parts of a further structural component consisting of at least one (meth)acryloyl-free compound containing groups which are reactive toward isocyanate groups.
These polyurethanes are used as binders in powder coatings. The blocking stability of these coating materials, however, is in many cases inadequate. EP 0 585 742 discloses that the blocking stability of coating materials based on these polyurethanes can be improved by blending them with solid, unsaturated polyesters. These polyesters are cheaper than the polyurethanes mentioned. By replacing a portion of the polyurethanes with the unsaturated polyesters, there is an overall reduction in cost for the powder coatings. With these powder coatings it is possible to coat even temperature-sensitive substrates, such as wood, wood materials and plastics, since they can be cured even at low temperatures by UV irradiation or by the addition of peroxides.
It is desirable for these powder coatings to flow just as well and to give surfaces that are just as smooth at this reduced baking temperature as conventional powder coatings. The binders must therefore exhibit good flow even at minimal processing temperatures. For this purpose, it is common to add flow additives, which markedly reduce the melt viscosity of the binder. However, these products do not take part in the curing reaction. This impairs the resistance of the coating to chemicals.
In a simultaneously filed U.S. patent application corresponding to German priority application P 44 32 645.9, this object is achieved by the addition of reactive, solid flow agents.
In powder coatings, it is not possible to reduce the melt viscosity to any desired extent, since in many cases the melting temperature decreases at the same time. During storage, the powder coatings thus formulated begin to agglomerate. The powder coating can then no longer be processed.